Methods of this type are currently known and used and are able to perform a fusion of pre-acquired images with real-time ultrasound images. Typically, these methods have a multimodal approach, such to combine the advantages of ultrasound acquisition with the advantages of other acquisition modalities, such as for example magnetic resonance imaging (MRI) or computerized axial tomography (CT).
Such technologies perform a fusion of highly detailed morphological information for example of MRI, with morphologic and above all hemodynamic information of ultrasound acquisition, for example by Color or Power Doppler modality.
Ultrasound imaging has some advantages with respect to MRI: it is safe and relatively inexpensive; the technology is widely available; it enables real-time imaging, with anatomical or morphological information (obtained by B-Mode imaging) and hemodynamic information (obtained by Color Doppler or Power Doppler technologies); it offers a good spatial resolution; it allows anatomical or vascular anomalies to be assessed.
On the other hand, ultrasound imaging is dependent on the ultrasound operator and on the radiologist who interprets the obtained images, it provides a small field of view, the resolution is reduced in case of bone tissue.
MRI has some advantages with respect to ultrasounds, such as a greater detail and contrast, a larger field of view, but at the same time it is more expensive and often it is not readily available.
In the light of such reasons, it is clear that multimodal fusion methods are gaining importance in clinical practice in order to fully exploit the advantages offered by the different modalities.
A more and more important application is the one related to fetal imaging. In such context, ultrasound imaging holds the supremacy as regards diffusion, simplicity and efficacy in use, and currently it is the main screening and diagnostic imaging modality as regards fetal imaging.
However, nowadays the use of MRI for prenatal diagnosis is continuously and considerably increasing. Literature, research papers and everyday clinical practice regarding this topic are constantly increasing. The reasons could be found in a real clinical added value and in the influence that MRI system manufacturers and suppliers are exerting on physicians and clinical institutions.
At the same time it is important to note that many of the malformations hard to be detected in ultrasound imaging relate to brain structures and blood vessels.
Fetal MRI is used, when available, as second imaging modality in case of particular pathologies detected or considered suspected by ultrasound analysis and/or laboratory analysis.
Therefore, in such scenario, it is logical to develop technologies able to fuse together information of the most common diagnostic imaging modality for obstetrics, that is ultrasound imaging (US), able to perform also hemodynamic studies by Color Doppler (CD) and Pulsed Wave Doppler (PW) with advanced diagnostic MRI modalities having a high spatial and contrast resolution and a high signal to noise ratio (SNR).
The fusion of US with MRI allows the operator to have all the advantages of the two modalities in real time, particularly the use of the hemodynamic US analysis, together with the MRI detailed analysis of brain and anatomical structures.
The fetal body district of major interest for the real-time fusion is the fetal head for two main anatomical/clinical reasons:
1. the cerebral structures are contained within the skull and, therefore, they are not affected by movements and/or structure distortions when the fetus moves;
2. the fetal head district can be affected by many pathologies, whose detection is based on minute structure changes and tiny differences with respect to normally developed fetal brain.
For such district therefore there is the need of an early diagnosis for many pathologies, which requires to contemporaneously analyse clinical information of the two imaging modalities.
Since ultrasounds (US) is the main diagnostic modality and since the MRI is used only when it is necessary, US-MRI fusion would be possible only by performing the real-time ultrasound acquisition after the acquisition of the MRI data. Practically speaking, usually after a first screening by ultrasound imaging, the fetal head MRI acquisition is performed and, therefore, the real-time ultrasound acquisition can be performed and the fusion of ultrasound and MRI images can be obtained.
The technologies for the fusion of the highly anatomical definition information of MRI with hemodynamic information of ultrasounds, allow what previously was analysed in two different modalities and time moments to be combined in real-time.
The main limitation of fetal multimodal fusion imaging is fetal movements, which are wide, frequent and independent with respect to the surface the probe rests on, that is the skin of the mother's belly. Fetal movements are highly probable both due to fetus autonomous movements and due to the high mobility of the womb, placenta and the amniotic sac of the mother.
Under such conditions, the currently known methods are not able to obtain satisfactory results, since the correlation of real-time ultrasound images and pre-acquired images obtained after the registration is irreparably lost upon the first considerable fetus movement.
Moreover often the pre-acquired images, for example 3D images acquired by MRI technology, have been acquired in moments different than ultrasound images and these moments can be such that, due to the rapid growth of the fetus, the sizes of the reference patterns or objects for the registration operations are very different and therefore they are difficult or inaccurate. Therefore under such conditions the advantages obtained by the fusion are at least partially compromised.